Tempest hot water indirect unvented cylinder confusion

Graham J W · 22 Apr 2026

Hi all,

Recently moved into a refurbished house and having issues with hot water delivery to "some" taps ?.

New 2023 installation of heating system and hot water. Heating fine via Vaillant ecoTech plus 630 coupled to a Tempest indirect 300L cylinder. Cylinder gets hot as required but :
Kitchen tap (nearest to cylinder in attached garage) takes ages before hot water comes through but the ground floor shower and sink ( in the next room) get hot water very quickly. Upstairs bathroom sink again takes ages for hot water to come through but the bath and the shower in the same room again get hot water quickly ?
Mains water pressure good so the cylinder should be pressurised ok.

Any thoughts appreciated

Johntheo5 · 23 Apr 2026

It's possible Xover from cold to hot, is the (balanced) cold supply taken off the unvented cylinder valve set like below??.

Graham J W · 23 Apr 2026

Good morning Johntheo5.
Thank you for replying. A valve such as that is installed and with the mains connected as your picture. There looks to be several connections from the outgoing side with one of them going to the cylinder. It has the expected 3bar marking on the valve which I believe is pre set by Telford cylinders. ( cold main inlet control group).

Johntheo5 · 23 Apr 2026

Can you shut off the mains cold into the unvented cylinder, should be a lever valve upstream of that valve group and then confirm that there is no cold water coming from any of the taps.
Are the (slow) kitchen and upstairs bathroom sink taps one lever mixers or are the hot and cold taps separate and then mixing when they leave the spouts?

Graham J W · 24 Apr 2026

Sorry for the late reply.
I shut off the incoming mains cold to the cylinder and all sinks in the house stopped running cold water. So I am assuming that the control group is maintaining 3bar pressure for both hot and cold to the house ?.
Taps to all sinks are lever mixers so the kitchen and bathroom taps (slow) are the same as the ground floor shower room tap (quick).

Johntheo5 · 24 Apr 2026

Do you have isol valves to the kitched tap and/or the bathroom tap?, if so close the hot one, put the lever valve to fully hot and see if any water comes out. Its very unlikely but your installer may have installed a isol lever valve on the cylinder HW outlet, if so, shut it and repeat the above.

Graham J W · 24 Apr 2026

Will do.

Graham J W · 24 Apr 2026

Hi, I isolated the hot water under the kitchen sink and turned on the hot tap and no water come out the tap. There is no isolation valve in the cylinder outlet pipework.
Question if I may. Could a booster pump be fitted in the HW outlet pipe from the cylinder ?. I appreciate that the cylinder is pressurised by the incoming mains at 3 bar but would the inclusion of the booster pump on the outgoing HW cause issues ?

Madrab · 25 Apr 2026

Graham J W said:
Could a booster pump be fitted in the HW outlet pipe from the cylinder ?. I appreciate that the cylinder is pressurised by the incoming mains at 3 bar but would the inclusion of the booster pump on the outgoing HW cause issues ?

You wouldn't normally use a pump to 'boost' the hot water from an unvented no and yes it can cause issues as the pump could be trying to draw more water than the combination valve's PRV, feeding the cylinder, is pre-set to supply. If the pressure/flow from all the hot water outlets is the same - it should be - then it can only be down to the length of run to each outlet, if cross mixing has been eliminated as an issue.

Graham J W · 25 Apr 2026

I think you are on the correct track in relation to pipe runs and lengths. I need to look into that further. Thanks for your advice on the booster pump and noted.

Johntheo5 · 26 Apr 2026

Graham J W said:
Kitchen tap (nearest to cylinder in attached garage) takes ages before hot water comes through but the ground floor shower and sink ( in the next room) get hot water very quickly. Upstairs bathroom sink again takes ages for hot water to come through but the bath and the shower in the same room again get hot water quickly

The kitchen tap takes ages even though nearest the HW cylinder but upstairs bath & shower do not, so one would think the kitchen tap run is the shortest?, pipe run etc'
Only other thing I can think of (if not X over which apparently it isn't) is that the kitchen tap and the other, even when turned to HW are mixing some cold water which may be due to a faulty or worn mixer but you can test that on the kitchen sink. Wait until the pipework etc cools down, isolate the cold supply to the kitchen tap then see if it still takes ages for the water to get hot.

Madrab · 26 Apr 2026

Johntheo5 said:
The kitchen tap takes ages even though nearest the HW cylinder but upstairs bath & shower do not, so one would think the kitchen tap run is the shortest?, pipe run etc'

It would make sense that the hot would be the shortest but it wouldn't be the first time I've seen a system retrofit to an out building where the old system was centrally in the house and the HW may now be taking a convoluted path to the get to the outlets.

If it's running an unvented I would also like to think that the hot and cold is balanced so ideally there wouldn't be an imbalance issue, unless the kitchen tap was running raw off the mains at a significantly higher pressure. That and if that mixer was passing the hot would never get up to temp, it would always be cooler, which doesn't seem to be the case, it's just taking longer to get up to temp.

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Tempest hot water indirect unvented cylinder confusion

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